FFR is an index of the functional severity of a coronary stenosis that is calculated from pressure measurements made during coronary arteriography and is defined as the distal blood pressure (behind a stenosis) relative to the proximal pressure (close to the Ostium) under hyperemic conditions (i.e. the ratio between the pressure after a lesion and the normal pressure). In other words, FFR expresses the maximal flow down a vessel in the presence of a stenosis compared to the maximal flow in the hypothetical absence of the stenosis. An FFR value is an absolute number between 0 and 1, where a value 0.50 indicates that a given stenosis causes a 50% drop in blood pressure, and facilitates diagnosis of the extent of a stenosis.
Computed Tomography Coronary Angiography (CCTA) is a non-invasive technique for the evaluation of coronary artery disease (CAD). The high negative predictive value in CAD detection positions CCTA as a non-invasive technique to rule out CAD in symptomatic patients with low to intermediate pre-test probability of disease. However, the literature indicates CCTA is limited in assessing hemodynamic significance of coronary lesions. Assessing hemodynamic significance from CCTA requires accurate segmentation of the coronaries to generate the three-dimensional model for flow simulations and a boundary conditions model that models the interface with the non-imaged vasculature. While many automatic and semi-automatic tools are available for generating the 3D model of the coronary tree from the CCTA data, accurate modelling of the boundary condition remains a significant challenge.
FFR-CT has the potential to improve CCTA specificity by adding hemodynamic significance assessment from CCTA data. It is known how to couple analytic models such as resistance, impedance and the Windkessel model to the boundaries of the truncated computational domain. However, these methods use constant parameters based on empirical measurements. In practice, there is a large variability between measurements of different individuals. Moreover, several studies show that the capillaries' resistance is auto-regulated to account for presence of stenosis in the parent coronary. An adaptive resistance FFR-CT model that accounts for auto-regulated changes in the capillaries' resistance due to the presence of coronary stenosis is described in PCT application PCT/EP2015/064168, filed on Jun. 24, 2015, and entitled “Apparatus for determining a fractional flow reserve value,” which is incorporated by reference herein in its entirety.
Collateral flow is an auto-regulation mechanism used by the body to prevent ischemia in case of coronary stenosis by creating new arterioles that support collateral blood flow to the potentially ischemic region. The literature indicates that even in the absence of obstructive coronary artery disease or in entirely normal hearts, there has been collateral flow to a briefly occluded coronary artery sufficient to prevent ECG signs of myocardial ischemia in 20-25% of the population studied. However, due to the small diameter of the collateral arterioles CCTA cannot directly depict the presence of collateral arterioles that support collateral blood flow. As a result, currently used boundary conditions models do not account for the presence of collateral flow. Unfortunately, this may cause inaccurate estimation of the FFR values.